Led switching arrangement

ABSTRACT

The invention relates to a switching arrangement for operating at least one LED, which switching arrangement is provided with input terminals ( 1, 2 ) for connecting a supply source, —output terminals ( 3, 4 ) for connecting the LED to be operated, —a first series circuit (I) between one of the input terminals ( 1 ) and one of the output terminals ( 3 ), including at least a self-inductance (L), a capacitor (C) and a diode (D), —a second series circuit (II) between the input terminals, including at least the self-inductance (L) and a switching element (S) which is alternately switched to a conducting state and a non-conducting state at a high frequency, —a third series circuit (III) between the output terminals, including the diode and an inductive winding (SW). According to the invention, the inductive winding forms a first winding (SW 1 ) of a transformer (T) which has a second winding (SW 2 ) that forms part of the first series circuit and which also has a connectiion point with the first winding.

[0001] The invention relates to a switching arrangement for operating aload, which switching arrangement is provided with

[0002] input terminals for connecting a supply source,

[0003] output terminals for connecting the load to be operated, a firstseries circuit between one of the input terminals and one of the outputterminals, including at least a self-inductance, a capacitor and adiode,

[0004] a second series circuit between the input terminals, including atleast said self-inductance and a switching element which is alternatelyswitched to a conducting state and a non-conducting state at a highfrequency, and

[0005] a third series circuit between the output terminals, including atleast said diode and an inductive winding.

[0006] A. switching arrangement of the kind referred to in theintroduction is known from U.S. Pat. No. 5,682,306. In the knownswitching arrangement, which is also known by the name of SEPIC (SingleEnded Primary Inductance Converter), the self-inductance forms a firstenergy storage element, and there will be a voltage across the capacitorsubstantially of the magnitude of an input voltage applied to the inputterminals. This type of converter appears to be suitable for operatingan LED array comprising at least one LED as the load. LED arrays arevery suitable for use as a light source, for example in a traffic lightsinstallation, inter alia on account of their low energy consunption incomparison with incandescent lamps which are suitable for use in suchtraffic lights installations and on account of the fact that they have amuch longer life than the incandescent lamps in question. In such anapplication, the switching arrangement and the LED array in question aregenerally supplied with power from a public mains as the power supplysource.

[0007] One drawback of the known switching arrangement is the occurrenceof a relatively high level of radio interference (EMI).

[0008] It is the object of the invention to provide a measure forreducing the EMI level.

[0009] In order to achieve that objective, a switching arrangement ofthe kind referred to in the introduction as the switching arrangementaccording to the invention is characterized in that the inductivewinding forms a first winding of a transformer which has a secondwinding that forms part of the first series circuit and which also has aconnection point with the first winding.

[0010] In this way it appears to be realized that a voltage change at aconnection point between the first and the second series circuit causedby the periodic switching of the switching element to a non-conductingstate is accompanied by a longer drift, which is highly conducive bothto a significant reduction of EMI and to a shift to lower frequencies.

[0011] Preferably, the second winding is directly connected to one ofthe output terminals by means of a snubber circuit. This helps toachieve a further reduction of the generation of interference signalswhich, surprisingly, has a negligible effect on the power transfer ofthe inductive winding.

[0012] An advantageous effect is realized if the winding ratio n1:n2between the first and the second winding is in accordance with therelation 1≧n1:n2≦0.66. The occurrence of inevitable leakage between thewindings is compensated by selecting a value n1 which is smaller thann2.

[0013] In an advantageous embodiment of the switching arrangementaccording to the invention, the first and the second winding togetherform a secondary winding of the transformer, which is provided with aprimary winding that forms part of both the first and the second seriescircuit. It has been found that this enables a significant furtherreduction of EMI to be achieved, because a high-frequency voltage signalacross the self-inductance caused by the high-frequency switching of theswitching element is effectively compensated in large measure via thevoltage induced in the primary winding. An optimum result can beachieved if the arrangement is configured in accordance with therelation

2π[(L1+Ls)C3]^(1/2)>δ,

[0014] wherein:

[0015] L1 is the magnitude of the self-inductance in H,

[0016] Ls is the magnitude of the self-inductance of the secondarywinding in H,

[0017] C3 is the capacitance of the capacitor in F, and

[0018] δ is the fraction in s of each switching period of the switchingelement during which the switching element is switched in itsnon-conducting state.

[0019] A further advantage can be realized if a bypass capacitor isarranged between the input terminals, which capacitor fimctions as abypass for the high-frequency ripple current signal generated by theprimary transformer winding.

[0020] The above and further aspects of the invention will be explainedin more detail hereinafter with reference to a drawing of the switchingarrangement according to the invention. In the drawing

[0021]FIG. 1 shows a diagram of a switching arrangement according to theinvention,

[0022]FIG. 2 shows a variant of the switching arrangement according toFIG. 1,

[0023] FIGS. 3A,B show current and voltage diagrams of a switchingarrangement according to the invention.

[0024] A switching arrangement according to the invention for operatingat least one LED is shown in FIG. 1, which switching arrangement isprovided with

[0025] input terminals 1,2 for connecting a supply source,

[0026] output terminals 3,4 for connecting the LED to be operated,

[0027] a first series circuit I between one of the input terminals 1 andone of the output terminals 3, including at least a self-inductance L, acapacitor C and a diode D,

[0028] a second series circuit II between the input terminals 1,2,including at least said self-inductance L and a switching element Swhich is alternately switched to a conducting state and a non-conductingstate at a high frequency,

[0029] a third series circuit III between the output terminals 3,4,including at least the diode D and an inductive winding. Furthermore, abuffer capacitor CB is arranged between the output terminals 3,4.

[0030] The inductive winding forms a first winding SW1 of a transformerT which has a second winding SW2 that forms part of the first seriescircuit and which also has a connection point VB with the first winding.Preferably, the second winding SW2 is directly connected to one of theoutput terminals by means of a snubber circuit 5. This helps to achievea further reduction of the generation of interference signals which,surprisingly, has a negligible effect on the power transfer of theinductive winding.

[0031] In an advantageous embodiment, a bypass capacitor BYC is arrangedbetween the input terminals 1,2.

[0032] In a variant of the switching arrangement according to theinvention which is shown in FIG. 2, the inductive winding forms asecondary winding SW of the transformer T which has a second winding PWthat forms part both of the first and of the second series circuit. Aconnection point between the switching element S and the primary windingPW forms a drain d of the switching element.

[0033] A practical embodiment of a switching arrangement according tothe invention as described in FIG. 1 is in particular suitable for beingoperated on a 230V, 50 Hz supply source. The switching arrangement issuitable for operating an LED array, for example forming part of atraffic lights installation. An array which is used in practice is ofthe type GR 090053224, made by Lumileds Lighting, which LED's emit greenlight. The circuit arrangement is capable of supplying the array with acontrolled current of a magnitude ranging between 540 mA and 750 mAduring operation, in dependence inter alia on the temperature of thearray.

[0034] In the switching arrangement, the self-inductance has a value L1of 3900 μH and the capacitor C has a capacitance C3 of 22 nF. The firstand the second winding SW1, SW2 of the transformer T comprise 36 and 54turns, respectively, and have a self-inductance of 85 μH and 455 μH,respectively. The buffer capacitor CB has a value of 330 μH. Theswitching element S is made up of a MOSFET type 2N60S5, made by Siemens.The value of the bypass capacitor BYC is 47 nF.

[0035] When a connected array as described above is being operated, theswitching element is alternately switched to a conducting and anon-conducting state at a high frequency of approximately 60 kHz. Thefraction δ of each switching period of the switching element duringwhich the switching element is switched in a non-conducting state rangesbetween 13 μs and 16 μs.

[0036] A practical embodiment of a switching arrangement as shown inFIG. 2 is in particular suitable for being operated at a supply voltageof 110V, 60 Hz. The secondary winding SW of the transformer T has aself-inductance Ls of 120 μH. The winding ratio of the primary windingPW and the secondary winding SW is 1:1. The primary and the secondarywinding comprise 62 and 56 turns, respectively. The first winding andthe second winding of the secondary winding each have 28 turns and aself-inductance of 50 μH. The switching element S is made up of a MOSFETtype IRF730, made by International Rectifier. The value of the bypasscapacitor BYC is 100 nF. The values of the other components correspondto those of the embodiment as shown in FIG. 1.

[0037]FIG. 3A shows, by way of illustration current and voltage curvesof a practical embodiment of the switching arrangement of FIG. 2, andFIG. 3B shows, by way of comparison, corresponding curves of a switchingarrangement as shown in FIG. 1. Both FIG. 3A and FIG. 3B show the driftin time of, respectively, the voltage (curve 100, 200) and the current(curve 101, 201) at the location of the drain d of the switching elementS in the form of a MOSFET. In the Figures, the horizontal axis is thetime axis. The current I and the voltage V are plotted on the verticalaxis.

[0038] In the case of FIG. 3B, the switching arrangement as described inFIG. 1 is concerned. In the case of FIG. 3A, the switching arrangementas described in FIG. 2 is concerned. In both cases the switchingarrangement is operated on a 120V dc supply voltage. A comparisonbetween the two shows that the use of the primary winding PW leads to asignificant reduction as regards the variation in the magnitude of thecurrent. Furthermore it appears that periodic switching of the switchingelement to a non-conducting state results in a reduction of thehigh-frequency ripple on the current. Both phenomena are highlyconducive to a further reduction of EMI.

1. A switching arrangement for driving at least one LED, which switchingarrangement is provided with input terminals for connecting a supplysource, output terminals for connecting the LED to be operated, a firstseries circuit between one of the input terminals and one of the outputterminals, including at least a self-inductance, a capacitor and adiode, a second series circuit between the input terminals, including atleast said self-inductance and a switching element which is alternatelyswitched to a conducting state and a non-conducting state at a highfrequency, a third series circuit between the output terminals,including said diode and an inductive winding, characterized in that theinductive winding forms a first winding of a transformer which has asecond winding that forms part of the first series circuit and whichalso has a connection point with the first winding.
 2. A switchingarrangement as claimed in claim 1, characterized in that the windingratio n1:n2 between the first and the second winding is in accordancewith the relation 1≧n1:n2≦0.66.
 3. A switching arrangement as claimed inclaim 1 or 2, characterized in that the arrangement is configured inaccordance with the relation 2π[(L1+Ls)C3]^(1/2)>δ, wherein: L1 is themagnitude of the self-inductance in H, Ls is the magnitude of theself-inductance of the secondary winding in H, C3 is the capacitance ofthe capacitor in F, and δ is the fraction in s of each switching periodof the switching element during which the switching element is switchedin its non-conducting state.
 4. A switching arrangement as claimed inclaim 1 or 2, characterized in that the inductive winding forms asecondary winding of the transformer which has a primary winding thatforms part of both the first and the second series circuit.
 5. Aswitching arrangement as claimed in claim 1, characterized in that abypass capacitor is arranged between the input terminals.